Back to EveryPatent.com
United States Patent |
5,153,516
|
Gopalsami
,   et al.
|
October 6, 1992
|
Solid-state NMR imaging system
Abstract
An apparatus for use with a solid-state NMR spectrometer includes a special
imaging probe with linear, high-field strength gradient fields and
high-power broadband RF coils using a back projection method for data
acquisition and image reconstruction, and a real-time pulse programmer
adaptable for use by a conventional computer for complex high speed pulse
sequences.
Inventors:
|
Gopalsami; Nachappa (Naperville, IL);
Dieckman; Stephen L. (Elmhurst, IL);
Ellingson; William A. (Naperville, IL)
|
Assignee:
|
The United States of America as represented by the United States (Washington, DC)
|
Appl. No.:
|
581957 |
Filed:
|
September 13, 1990 |
Current U.S. Class: |
324/309 |
Intern'l Class: |
G01R 033/20 |
Field of Search: |
324/300,307,308,312,313,314,318,322
128/653 R,653 A
|
References Cited
U.S. Patent Documents
4859949 | Aug., 1989 | McKenna | 324/318.
|
5041789 | Aug., 1991 | Keller et al. | 324/318.
|
Other References
Listerud et al., "NMR Imaging of Materials", Analytical Chemistry, vol. 61,
No. 1, p. 23a, Jan. 1, 1989.
"New Micro-Imaging Accessory for AM and MSL Systems", Bruker Instruments,
Inc.
|
Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Cordell; Helen S., Albrecht; John M., Moser; William R.
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
The U.S. Government has rights in this invention pursuant to Contract No.
W-31-109-ENG-38 between the U.S. Department of Energy and the University
of Chicago.
Claims
The embodiments of this invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An apparatus for use with a conventional solid-state NMR spectrometer,
comprising:
an imaging probe which generates high linear gradient fields,
and a solid-state pulse programmer which when installed in a microcomputer
is programmable and provides control pulse sequences for excitation of
said imaging probe and for shaping of RF pulses and on board data
acquisition and memory.
2. The apparatus of claim 1 wherein said imaging probe generates high
linear gradient fields in excess of 50 G/cm at 10% duty cycle and permits
emplacement of RF coils for samples which are 28 mm wide.
3. The apparatus of claim 2 wherein said imaging probe includes a
saddle-type y gradient coil with coil span of 120 degrees, an identical x
gradient coil rotated 90 degrees with respect to said y gradient coil, and
a z gradient which includes a pair of Maxwell coils wound in opposite
directions.
4. The apparatus of claim 3 wherein said imaging probe includes an RF coil
designed in a saddle-type configuration with a span of 120 degrees and the
height of said RF coil is approximately equal to the diameter of the coil
form.
5. The apparatus of claim 1 wherein said solid-state pulse programmer
allows random access jumping to any address and user definition of
synchronous and asynchronous digital input/output configurations.
6. The apparatus of claim 5 wherein said solid-state pulse programmer
permits data acquisition using both external and synchronous internal
triggering.
7. The apparatus of claim 6 wherein said solid-state pulse programmer
includes acquisition memory chips which are at least 32 K deep.
8. The apparatus of claim 7 wherein said solid-state pulse programmer
includes ports for at least 12 B user definable, synchronous data
input/output lines.
9. An NMR imaging system with solid-state components providing high
gradient field strength and narrow line widths, comprising:
a magnet for generating a magnetic field,
an imaging probe in said magnet, said imaging probe including
amplifier-driven gradient coils generating high linear gradient fields in
excess of 50 G/cm at 10% duty cycle, and an RF-powered coil for
transmitting RF power to a sample enclosed in said imaging probe,
spectrometer means for digitizing and processing NMR data generated from
sample,
a programmable solid-state pulse programmer installed in a microcomputer,
said pulse programmer sequencing gradient pulses to amplifiers driving
said gradient coils, shaping RF pulses transmitted to said sample, and
acquiring and storing NMR data generated by said sample and said
spectrometer, and
imaging means for image reconstruction and processing of said NMR data.
10. The imaging system of claim 9 wherein said pulse programmer includes a
mother board with solid-state implementations of delay counting, address
generation, and internal and external triggering.
11. The imaging system of claim 10 wherein said mother board provides
real-time control of multiple memory boards with user definable,
synchronous and asynchronous digital input/output configurations.
12. The imaging system of claim 11 wherein said imaging probe uses the back
projection method for data acquisition and image reconstruction.
13. The imaging system of claim 12 wherein said imaging probe includes a
saddle-type y gradient coil with coil span of 120 degrees, an identical x
gradient coil rotated 90 degrees with respect to said y gradient coil, and
a z gradient which includes a pair of Maxwell coils wound in opposite
directions.
Description
BACKGROUND OF THE INVENTION
Researchers have become increasingly aware of the potential for nuclear
magnetic resonance (NMR) imaging, also called magnetic resonance imaging
(MRI), for materials science applications. NMR uniquely allows
noninvasive/nondestructive mapping of the internal chemical and physical
properties of materials and provides quantitative information on the
chemical microstructure of materials. (See further, "NMR Imaging of
Materials, Analytical Chemistry, Vol. 61, No. 1, page 23a).
The current invention contemplates use of NMR imaging for any nuclei
normally NMR imaged, including proton, carbon, sodium and phosphorus, in a
variety of materials including polymers and geologic materials, such as
coal, and is particularly useful in imaging structural ceramics.
Hydrogen nuclei (protons), which are present in organic binders of green
ceramic bodies, are the most sensitive NMR-active nuclei that can be used
for MRI studies. With proper set up of an imaging experiment (choosing
pulse sequences), NMR signal intensity (gray scale levels) can be made
proportional to the amount of organics present in a local volume of
interest (voxels). Like x-ray computed tomography, MRI can be used to
provide two dimensional tomographic images of selected slices and as a
quantitative technique for determining spatial distribution of organics
within a green body.
Medical MRI systems based on solutions NMR (e.g. using water molecules
abundant in biological subjects) are inadequate for the imaging of
organics within a green body primarily because of differences between the
line widths of NMR spectra. For example, the line width of proton spectra
from organics in green ceramic materials is about 2500 Hz, compared to a
few Hz in biological systems. Because linear gradient fields are used in
NMR imaging to frequency-label spatial positions, the gradient strength
required to resolve two positions in space must be enough to ensure that
the difference in resonance frequencies between these two positions is
greater than the line widths of the resonances. The imaging of ceramics
with a spatial resolution of 100 .mu.m, for example, would require a
gradient strength of 50 G/cm, which is beyond that normally found in
medical systems.
Another difference is in imaging technique. While spin-warp imaging is used
in medical MRI systems, back projection is the method of choice for
materials with short spin-spin relaxation time, T.sub.2 (large line widths
correspond to short T.sub.2). This method allows NMR response (FID) to be
detected immediately after RF excitation, thus preserving maximum signal
intensity. Back-projection, however, poses more stringent specifications
on probe design, requiring (1) high uniform gradient and RF fields, (2)
well-balanced gradient fields between orthogonal axes, and (3) strict
alignment of static, gradient, and RF fields with respect to the center of
the sample space. Also, the RF bandwidth must be great enough to span the
entire range of frequencies produced by the gradient fields.
Modern NMR spectrometers and imaging units typically require some degree of
digital control over a large number of subcomponents including RF
amplifiers, RF receivers, frequency synthesizers, magnetic field gradient
controllers, attenuators, filters and phase shifters. Demanding
experimental sequences may require 100 nsec resolution, submicrosecond
control, and output (perhaps thousands of commands over several seconds)
to more than 250 digital control lines.
State of the art pulse programmers incorporate a pulse sequence to be
expressed when the experiment is performed. The present invention provides
an apparatus which provides for easy implementation with a variety of
commonly available host computers and incorporates digital control ability
to an expandable number of control/data acquisition lines, permitting
on-site programming of complex, high speed pulse sequences with a large
number of unique instructions for better resolution in pulse shaping.
It is therefore a primary object of this invention to provide a solid state
NMR imaging system with high gradient field strength and the ability to
narrow line widths.
In the accomplishment of the foregoing object, it is another important
object of this invention to provide an NMR imaging system using back
projection techniques while providing high uniform gradient and RF fields,
well-balanced gradients fields between orthogonal axes, and strict
alignment of static, gradient, and RF fields with respect to the center of
the sample.
It is another important object of this invention to provide a flexible
solid state NMR imaging accessory for operation with conventional
wide-bore solid-state NMR spectrometers.
It is a further object of this invention to present an NMR imaging probe
which permits use of alternative RF coils for varying sample sizes for
increased efficiency and signal-to-noise ratio.
Finally, it is an object of the present invention to present a PC-based
pulse programmer which is programmable to synchronize control pulses for
gradient coils with a spectrometer radio frequency and which is usable
with commonly available heat computers of choice.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, this invention comprises an NMR
imaging system, and more specifically, a flexible solid state imaging
accessory for operation with conventional wide-bore solid-state NMR
spectrometers. The imaging accessory includes a special imaging probe
using the back projection method for data acquisition and image
reconstruction, and a pulse programmer adaptable for use by conventional
computers, useful for complex, high speed pulse sequences. In a current
embodiment (designed for an 89 mm bore magnet) the imaging system uses
samples up to 28 mm wide and can generate high linear gradient fields in
excess of 50 G/cm at 10% duty cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated in the accompanying drawings where:
FIG. 1 is a block diagram of the solid-state NMR imaging system of the
present invention.
FIG. 2 is a schematic drawing of the imaging probe included in the imaging
system.
FIGS. 3a and 3b are schematic drawings of the probe's gradient coil
geometry.
FIG. 4a and 4b are graphic plots of simulated and experimentally determined
magnetic fields for the gradient coils.
FIGS. 5a and 5b are schematic drawings of the RF coil geometry and of a
tuning and matching circuit, respectively, and FIG. 5c a graphic plot of
RF coil resonance.
FIGS. 6a and 6b are block diagrams of the pulse programmer mother board
(main controller board) and a memory I/O board, respectively.
FIG. 7a is a logic diagram of the mother board operation during run time,
and FIG. 7b is a logic diagram of run time flag decoding.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram of the NMR imaging system of the present
invention. The system is comprised of spectrometer 10, a special imaging
probe 12, gradient amplifiers 22, a unique pc-based pulse programmer 24,
image reconstruction computer 26, and PC-based graphics work station 28
for image processing and display.
Pulse programmer 24 consists of PC-based software and hardware which uses
trigger pulses from spectrometer 10 to provide control pulses to x, y, z
gradient amplifiers 22. Pulse programmer 24 (described in greater detail
below) has flexibility to generate a variety of pulse protocols along with
RF pulse shaping and is therefore adaptable to the spectrometer of choice.
In the preferred embodiment, three Techron power amplifiers 22 (DC-45 kHz;
2 kVA) drive gradient coils in the constant-current mode based on gradient
pulses from pulse programmer 24. NMR data are acquired on a Bruker Model
CXP-100 spectrometer and then transported from an Aspect 3000 computer to
a VAX 8700 computer via Ethernet for image reconstruction and processing.
Image reconstruction computer 26 for reconstruction of NMR back projection
images uses software which includes features found in prior art software
for NMR spectroscopy, tomographic reconstruction, and image display
enhancement. NMR spectroscopic features include frequency and time domain
filters, and FFTs for data conversion to frequency domain. Computed
tomographic reconstruction of images occurs from spectroscopic data in the
frequency domain. The reconstruction software implements filtered back
projection algorithms with a choice of a wide variety of filters
(convolution kernels).
FIG. 2 is a schematic drawing of imaging probe 12. In the preferred
embodiment, probe 12 is designed to be used in an 89 mm vertical bore,
2.35-T superconducting magnet (not shown) and can accommodate samples up
to 28 mm in diameter. Probe 12 includes: RF coil 14 wound on RF coil form
32, gradient coils 16, 18 and 20 (not shown) wound on gradient coil form
34, RF shield 15, tuning capacitors 36, tuning rods 38, and thermocouple
40.
To improve the filling factor of RF coil 14, probe 12 allows emplacement of
different sized RF coils (<30 mm), depending on sample size. At high field
strengths, gradient coils 16, 18 and 20 will generate considerable heat
because of resistive losses; hence, thermal dissipation is enhanced using
cooling air 42. As a safety feature, thermocouple 40 monitors the
temperature, and signals a controller (not shown) to shut down the system
when temperatures are excessive. Extraneous RF fields are, to a large
extent, precluded from entering detection circuitry by RF shield 15, thus
improving signal-to-noise ratio.
As depicted in schematic drawing FIG. 3a, y gradient coil 18 is designed in
a saddle-type configuration, with a coil span of 120 degrees. The x
gradient coil 16 (not shown) is identical but rotated by 90 degrees. As
shown in FIG. 3b, a pair of Maxwell coils wound in opposite directions are
used for z gradient 20.
Coils 16, 18 and 20 were designed for optimal field strength without loss
of linearity and uniformity of field in a maximum sample space for the
given bore using a finite-element analysis of fields with the computer
code TOSCA.TM. from Vector Fields. In addition, winding and substrate
materials were selected for high heat transfer characteristics and
strength. For the given bore, high linearity and field strength are
achieved using coils wound in two layers with 12 turns each of AWG 22
copper wire and the geometry depicted in FIG. 3b.
As depicted in FIGS. 4a and 4b, for all three gradients simulated fields
show good agreement with measured fields. Testing was performed by driving
with constant currents from a Techron power amplifier and measuring
magnetic fields along x, y, and z axes with a Hall probe. Projected
gradient strength of the fields is in excess of 50 G/cm at the maximum
coil rating of 20 A at a 10% duty cycle.
FIG. 5a is a schematic drawing of the geometry of RF coil 14. An RF coil
must efficiently transmit RF power to the sample volume to excite nuclear
spins and to detect precessing nuclear magnetization with a high
signal-to-noise ratio. The main design requirements of the RF coil are
that it should resonate at the desired operating frequency with a high Q,
produce a homogeneous magnetic field transverse to the main magnetic axis,
and provide a good filling factor. The saddle-type RF coil design shown in
FIG. 5a satisfies these requirements in the frequency range of interest
(100 MHz) and also allows convenient loading of samples. A single-turn
coil was wound on a 30-mm outside diameter NMR glass tube, and its
resonant characteristics were tested. As in the saddle coil design of the
gradient coils, the span of the coil is 120 degrees; the height of the
coil is nearly equal to the diameter of the coil form.
A tuning and matching circuit 50 shown in FIG. 5b was constructed so RF
coil 14 can be resonated at an operating frequency of about 100 MHz and
its impedance matched to 50 .OMEGA., which is the characteristic impedance
of the coaxial cable which connects the coil to the transmitter and
receiver circuits of the spectrometer.
FIG. 5c shows the resonant behavior of the circuit with RF coil 14
operating at frequencies in the desired range; a quality factor (Q) of 239
was obtained with no sample inside the coil.
Designed as a set of circuit boards depicted in block diagrams FIGS. 6a and
6b, pulse programmer 24 includes the following general features:
(1) Random access jumping to any address, allowing implementation of
complex pulse sequences,
(2) On board data acquisition allowing jumper selectable data input on the
memory I/O boards,
(3) Expandability allowing a flexible configuration up to a maximum of 892
digital I/O control/data acquisition lines,
(4) Deep memory buffer (32 K deep memory) allowing implementation of
complex pulse sequences that require a large number of unique
instructions,
(5) High speed memory allowing 100 nsec access time, with a minimum state
duration of 200 nsec, and
(6) General purpose CPU bus allowing easy implementation with a variety of
commonly available host computers.
In FIG. 6a, pulse programmer mother board 60 provides real-time control of
memory I/O boards shown in Fib. 6b and includes five main functions: delay
counter implemented by delay clock memory 63 and delay clock 64; address
generator implemented by jump index counter 65, jump indirect memory 66
and main address counter 67; control logic implemented by real time
control memory 68, real time control logic 69, trigger control logic 70
and main control latch 71; external trigger input 72; and computer
interface implemented by bus transceiver 73 and tristate line driver 74.
System timing created by the 32 bit synchronous delay counter operates at
10 MHz, giving a resolution of 75 nsec, and a minimal pulse width of 200
nsec. In addition to the 10 MHz internal frequency standard, clock input
75 provides an external 10 MHz input to lock pulse programmer 24 to the
frequency reference of spectrometer 10. The use of 32 K deep memory chips
(easily increasable to 128 K) for both control functions and memory I/O
boards allows the implementation of large complex pulse sequences. Trigger
input 72 is both maskable and resettable, allowing multiple trigger
requests within a pulse sequence.
Jumping is implemented through the technique of Indexed Indirect Addressing
(IIA). This method of jumping incorporates an indexed memory lookup table
which contains the new jump addresses. When the "Jump" instruction is
encountered, the base address is loaded from the lookup table, followed by
incrementing of the lookup table index. This method avoids loop nesting
restrictions, and allows jumping to virtually any address. Thus, the IIA
method is a true random access jumping method.
In FIG. 6b, memory board 80 consists of 128 bit wide 32 K deep user
definable, synchronous digital I/O lines. Data I/O lines are grouped as
eight 16 bit data ports with standard 40 pin header output. Data port 81
as depicted in FIG. 6b represents six of the eight bit data ports (five
iterations of data port 81 are not shown). Two sixteen bit data ports 82
and 83 (thirty two of data lines) can be individually configured as either
input or output ports as well as for external or synchronous internal
triggering using input/output logic 84 and 85, respectively. This
flexibility allows data acquisition without the additional hardware or the
timing problems associated with direct computer controlled data
acquisition. All memory chips 86 are static, low power, 100 nsec access
time NMOS allowing data acquisition at the maximum controller rate of
every two clock cycles or 200 nsec intervals. A maximum of seven memory
boards can be daisy chained to the controller shown in FIG. 6a, allowing a
total of 896 user definable I/O bits.
Operation of the NMR imaging system of the present invention is divided
into two modes of operation : "Execute" and "Load". The active mode is
determined by the host CPU and latched into the EXE/LOAD flag of the Main
Control Latch.
In the LOAD mode the controller's main timer is disabled, and the internal
data bus is activated. The host CPU then programs the system's latches,
clocks and memory buffers with the appropriate bit patterns for the
desired pulse sequence. Data transfers occur via the 8 bit data bus at the
host's data bus rate.
A flow diagram of the control sequence in "Execute" mode is shown in FIGS.
7a and 7b. When the NMR experiment begins, the host CPU sets the "EXE"
flag in the main control latch, causing both the deactivation of the
internal data bus, and the reactivation of the main system clock. The 32
bit system dwell clock increments until the overflow occurs at which time
a read pulse is generated. The result is the synchronous expression of
data from the 32 bit dwell clock memory buffer, the real-time control
logic buffer, and the memory I/O boards. If the real-time logic control
latch's "Jump" flags are not set, the memory address generator is
sequentially incremented. With the "Trigger" and "Stop" flags unset, the
experiment will continue with the output of data from the next memory
address.
Testing and servicing the "Jump", "Stop", and "Trigger" flags occurs
concurrently as indicated in FIG. 7b. When the "Jump" flag is encountered,
a second read pulse is generated, causing the jump address to be read out
of the jump indirect address memory, and latched into the memory address
counter. The jump indexed counter is then incremented. If the "Trigger"
flag is encountered, the main system clock is disabled, and reactivated
when the external trigger occurs. The "Stop" flag completely stops the
pulse programmer, by causing the main system clock to be permanently
disabled.
The foregoing description of a preferred embodiment of the invention has
been presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise form
disclosed, and obviously many modifications and variations are possible in
light of the above teaching. The embodiments described explain the
principles of the invention and practical applications and should enable
others skilled in the art to utilize the invention in various embodiments
and with various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be defined by
the claims appended hereto.
Top